Spiraling giants: witnessing the birth of a massive binary star system

Astronomers have found a molecular cloud that is collapsing to form two massive protostars that will eventually become a binary star system. This observations show that binary stars form together.

ALMA’s view of the IRAS 07299-1651 star-forming cloud and the massive binary system at its center. The background image shows dense, dusty streams of gas (shown in green) that appear to be flowing towards the center. Blue indicates gas moving towards us and red indicates gas moving away from us. The inset image shows a close up view of the massive forming binary. The brighter, primary protostar is moving toward us and the fainter, secondary protostar is moving away. The blue and red dotted lines show calculated orbits for the primary and secondary spiraling around their common center of mass (marked by the cross).

Copyright : RIKEN, ALMA (ESO/NAOJ/NRAO), Zhang et al.

Astronomers have found a molecular cloud that is collapsing to form two massive protostars that will eventually become a binary star system. This observations show that binary stars form together.

Scientists know that most massive stars have a smaller star orbiting them. But it has been unclear whether these binary stars are born together or if the stars pair up later. It is difficult to observe the formation of a binary system directly because the protostars, objects condensing to form stars, are hidden inside clouds of gas and dust. Radio waves can escape from within the clouds but previous radio telescopes have lacked the resolution needed to see the details of the formation process.

A team including scientists from the RIKEN in Japan, Chalmers University of Technology in Sweden, and the University of Virginia in the USA used ALMA (Atacama Large Millimeter/submillimeter Array) to observe the radio waves from IRAS 07299-1651, a star-forming cloud about 5,500 light-years away in the constellation Puppis.

Thanks to ALMA’s unprecedented spatial resolution, the team was able to see that already at this early stage, the cloud contains two objects, a massive “primary” central protostar and another “secondary” protostar. The two forming stars are separated by a distance of about 27 billion kilometers (about 6 times the distance between the Sun and Neptune). The protostars are currently orbiting each other with a period of at most 600 years, and have a combined mass at least 18 times that of our Sun.

Another finding of the study was that the binary stars are being nurtured by a shred gas disk fed by the collapsing cloud. This suggests the binary formed from fragmentation of the disk originally around the primary. This will allow the initially smaller secondary protostar to “steal” infalling matter from its sibling and eventually they should emerge as quite similar “twins.”

Yichen Zhang, the leader of the team, explains, “Our observations clearly show that the division into binary stars takes place early on, while they are still in their infancy. What is important now is to look at other examples to see whether this is a unique situation or something that is common for the birth of all massive stars.”